Universal Mobile Telecommunications System (UMTS) is a 3rd Generation (3G) asynchronous mobile communication system operating in Wideband Code Division. Multiple Access (WCDMA) based on European systems, Global System for Mobile communications (GSM) and General Packet Radio Services (GPRS). In the 3GPP release 99, the radio network controller (RNC) controls resources and user mobility. Resource control includes admission control, congestion control, and channel switching which corresponds to changing the data rate of a connection. A dedicated radio connection is carried over a dedicated channel DCH, which is realized as a DPCCH (Dedicated Physical Control Channel) and a DPDCH (Dedicated Physical Data Channel).
The Long Term Evolution (LIE) of UMTS is under discussion by the 3rd Generation Partnership Project (3GPP) which standardized UMTS. The objective of the LTE work is to develop a framework for the evolution of the 3GPP radio-access technology towards a high-data-rate, low-latency and packet-optimized radio-access technology. In particular, LTE aims to support services provided from the packet switched (PS)-domain. A key goal of the 3GPP LTE technology is to enable high-speed packet communications at or above about 100 Mbps.
A mobile radio terminal, often referred to as a user equipment (UE), in an idle state monitors system information broadcast by base stations within range to inform itself about “candidate” base stations in the service area. When a mobile terminal needs access to services from a UMTS radio access network, it sends a request over a random access channel (RACH) via a suitable base station, typically a base station with the most favorable radio conditions. Because the uplink propagation conditions are usually only approximately known, the mobile terminal gradually increases its transmission power over the RACH until either the base station acknowledges the message or a predetermined number of unsuccessful access attempts has been reached. But assuming the mobile terminal is admitted access, a radio communications connection or link via the most suitable base station is initiated towards the mobile terminal if there are available radio resources. Uplink coverage by the base station is thus a necessity for successful random access.
There is a trade-off between uplink coverage and uplink-enabled peak transmission rates over the radio interface. This trade-off is even more pronounced in systems that provide enhanced uplink communications supporting higher uplink data rates than typical dedicated channels. The uplink radio resources in a cell coverage areas are limited by the rise over thermal (RoT) that the cell can tolerate. The RoT is the total received power at the base station divided by the thermal noise in the cell, and the cell coverage is limited by a maximum RoT. The maximum RoT is either determined based on coverage requirements and/or uplink power control stability requirements. When only one UE is transmitting over an uplink connection in the cell, both power control stability and coverage are minor issues because the uplink interference is likely to be dominated by the power generated by this UE. In this situation, a higher maximum RoT may be used to allow a higher signal-to-interference ratio Ec/Io, which enables higher uplink bit rates. But in order to use the higher uplink bit rates, the UE connections have to provide high Ec/Io, which implies high RoT.
Cells operating at high RoT unfortunately have limited coverage. Higher RoTs may make it difficult or even impossible for mobile terminals to successfully complete random access from some parts of the cell service area. The RACH preamble may not be detected at these high RoT when sent from certain parts of the service area. Furthermore, the gradual power increase by mobile terminals requesting access may generate significant interference in the cell, which decreases the signal-to-interference ratio Ec/Io, which negatively impacts the uplink mobile terminal data rates. Without regulation, mobile terminals may even request higher uplink data rates and be permitted to transmit at higher uplink data rates even though they may not be capable of or will even benefit from being permitted to transmit at higher uplink data rates.
Neither is it practical to lower the RACH preamble threshold because this will lead to many erroneous preamble detections caused only by thermal noise. In many cases, a lower RACH preamble threshold also results in legitimate RACH transmissions at too low power levels, which will not be decoded correctly, and thus, must be retransmitted. Moreover, the short time between a received preamble and when a RACH acquisition indicator is expected means there is very limited time for processing before it must be determined whether a preamble was sent at a sufficient power level.